Wireless communication systems are widely known in which base stations (BSs) perform radio communication with user equipments (UEs) located within range of the said BSs. The area covered by one or more (typically three) BS(s)—i.e. the geographical region serviced by that/those BS(s)—is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to cover a wide geographical area more or less seamlessly with adjacent cells. There is a constant need to increase the capacity of such systems and to improve the efficiency of resource utilisation in order to accommodate more users, more data-intensive services and/or higher data transmission rates (i.e. to improve the system transmission capacity).
Orthogonal Frequency Division Multiplexing (OFDM) is one known technique for transmitting data in wireless communication systems. An OFDM-based communications scheme divides data symbols to be transmitted among a large number of frequency subcarriers, hence the description “frequency division multiplexing”. Data is modulated onto a subcarrier by adjusting its phase, amplitude or both. The “orthogonal” part of the term OFDM refers to the fact that the spacings of the subcarriers in the frequency domain are chosen so as to be orthogonal, in a mathematical sense, to the other subcarriers. In other words, the sinusoidal waveforms of each subcarrier are eigenfunctions of a linear channel, with the peak of each sinusoid coinciding with a null of every other sinusoid. This can be achieved by making the subcarrier spacing a multiple of the reciprocal of the symbol period.
When individual subcarriers or sets of subcarriers are assigned to different users or user equipments (UEs) in the system, the result is a multi-access system referred to as Orthogonal Frequency Division Multiple Access (OFDMA). The term OFDM is often used to include OFDMA, and vice versa. The two terms may therefore be considered interchangeable for present purposes. By assigning distinct frequency resources (i.e. distinct orthogonal subcarriers) to each UE in a cell, OFDMA can help to reduce interference among the UEs within a given cell.
A means by which basic OFDM schemes are increasingly being improved for increased data rates in wireless communication systems is through the use of so-called multiple-input multiple-output (MIMO) schemes. MIMO schemes employ multiple antennae at the transmitter and/or at the receiver (generally at both) to enhance the data capacity achievable between the transmitter and the receiver. By way of example, in a basic 2×2 MIMO configuration there are two antennae at the transmitter and two antennae at the receiver. Likewise, a basic 4×4 MIMO configuration contains four antennae at the transmitter and four antennae at the receiver. There is no need for the transmitter and receiver to employ the same number of antennae. Typically, a BS in a wireless communication system will be equipped with more antennae in comparison with a UE (which may often be, for example, a mobile handset), owing to differences in power, cost and size limitations.
The term MIMO channel (or simply “channel”) is commonly used to describe the frequency response of the transmitter-receiver radio link in a MIMO scheme. The MIMO channel may be represented mathematically as a matrix H, the individual elements of which represent the channel characteristics (for example, channel frequency response) for transmitting signals from one particular transmitting antenna to one particular receiving antenna. For example, the element Hb,a of matrix H would represent the channel characteristics for transmitting signals from the ath transmitting antenna of a BS to the bth receiving antenna of a UE.
It should be noted that, despite the name “multiple-input multiple-output”, MIMO systems can operate (and indeed provide benefit) even if one of the transmitter and the receiver has only one antenna. In fact, MIMO systems might technically be said to operate even where the transmitter and the receiver both have only one antenna, although this situation might be considered a special (degenerate) case because the MIMO channel would then be represented by a scalar rather than a matrix and a number of the benefits otherwise achievable using MIMO may not be possible.
Unfortunately, achieving high spectral efficiency (or high transmission capacity) by using MIMO schemes in a wireless communication system involves a number of requirements. These include, for example, feedback of channel state information (CSI) from UEs to BSs (this is necessary to provide the BSs with information regarding time-varying changes in the channel and so that the BSs can perform pre-coding, link adaptation etc accordingly). Something else which is so advantageous that it may be thought of as a requirement is the use of a frequency reuse factor of one. However, this level of frequency reuse in particular inevitably leads to inter-cell interference experienced by UEs, particularly those UEs near cell edges (so-called “cell-edge users”). This inter-cell interference may also be referred to as “other cell interference” or OCI (the terms “inter-cell interference” and OCI may be considered synonymous) and can significantly degrade system spectral efficiency.
To elaborate, inter-cell interference or OCI may arise, for example, because the frequency resources (i.e. the carriers and subcarriers) utilised by base stations transmitting data to UEs in one cell are identical to the frequency resources utilised by nearby base stations transmitting data to UEs in an adjacent cell. In other words, there is 1:1 frequency reuse between adjacent cells. As a result of this, for a cell-edge user UE, the distance to a base station currently serving that UE may be roughly the same as, or only marginally different to, the distances to base stations that are in adjacent cell(s). Consequently, from the point of view of that cell-edge user UE, the signal strength received from the serving base station may be only marginally stronger than, or approximately the same as, the signal strength from the base stations in the adjacent cell(s). And because common frequency resources are used in adjacent cells, signals being transmitted in the adjacent cells can often interfere with data being transmitted to the cell-edge user UE in its cell.
In recent years, a technology referred to as Base Station Cooperation (BSC) has been proposed to address the problem posed by OCI. Previous proposals relating to BSC have generally suggested coordinating transmissions between BSs of adjacent or nearby cells to eliminate or reduce OCI. One form of downlink scheme proposed for BSC is commonly referred to as “Joint Transmission”.
In Joint Transmission, data to a single UE is simultaneously transmitted from multiple transmission points (i.e. from multiple BSs) in a cooperative manner that is adapted to improve the received signal quality at the UE and cancel interference caused by OCI. The Joint Transmission category of BSC enables “networked” MIMO transmissions by setting up cooperation between geographically separated BSs. The present invention is primarily concerned with BSC schemes which fall into this Joint Transmission category, and hereafter the term “BSC” refers to this.
It has been shown that BSC has the potential to be enormously effective in terms of OCI suppression and thus to improve overall system throughput. See, for example, the following document which may be considered a useful reference:                H. Zhang and H. Dai, “Cochannel interference mitigation and cooperative processing in downlink multicell multiuser MIMO networks,” EURASIP Journal on Wireless Communications and Networking 2004:2, pp. 222-235 (hereafter “Zhang and Dai”)        
Zhang and Dai discloses, among other things, a number of joint transmission schemes for BSC, including so-called joint transmission Minimum Mean Square Error (MMSE), so-called joint transmission Zero Forcing (ZF) and so-called joint transmission Null Space Decomposition (NSD). Zhang and Dai also discusses non-BSC transmission schemes, which may be referred to for convenience as “conventional” transmission schemes.